Savvy sensor

Using lasers and tuning forks, US researchers have developed a chemical weapon agent sensing technique that could exceed current and emerging defence and homeland security chemical detection requirements.

Using lasers and tuning forks, researchers at Pacific Northwest National Laboratory have developed a chemical weapon agent sensing technique that could exceed current and emerging chemical detection requirements. The technique, called Quartz Laser Photo-Acoustic Sensing (QPAS), is now ready for prototyping and field-testing.

PNNL, a US Department of Energy national laboratory, has demonstrated QPAS’s ability to detect gaseous nerve agent surrogates. In one test, researchers used diisopropyl methyl phosphonate (DIMP), which is a chemical compound similar to sarin. QPAS detected DIMP at the sub-part-per-billion level in less than one minute. The miniscule level is similar to letting one drop of liquid DIMP evaporate into a volume of air that would fill more than two Olympic-size swimming pools.

‘QPAS is an extremely sensitive and selective chemical detection technique that can be miniaturized and yet is still practical to operate in field environments,’ said Michael Wojcik, a research scientist at PNNL. ‘The laser, tuning fork and other technology needed for QPAS are so simple, and yet robust, that further development is a low-risk investment, and we’re eager to take it to the next level.’

The instrument is based on Laser Photo-Acoustic Sensing (LPAS) and infrared Quantum Cascade Lasers (QCLs). LPAS is a sensitive form of optical absorption spectroscopy, where a pulsed laser beam creates a brief absorption in a sample gas, which in turn creates a very small acoustic signal. A miniature quartz tuning fork acts as a microphone to record the resulting sound wave.

PNNL researchers paired multiple QCLs with the tuning forks, allowing simultaneous examination of a single sample at many infrared wavelengths. Nearly every molecule has unique optical properties at infrared wavelengths between three and 12 micrometers, and QCLs provide access to any wavelength in this region.

‘Because of this access and the fact that QPAS is almost immune to acoustic interference, we have potential for extraordinary chemical sensitivity and selectivity,’ Wojcik said.

QPAS’s small components are said to represent a major advance over previous LPAS measurement methods. Historically, LPAS instruments were physically large, often measuring a metre or more in length. The entire arrangement was prone to interference from external sound and vibration.

In the QPAS technique, several QCLs can fit on a 3 x 3 millimetre chip. And the tuning forks are identical to the kind used in wristwatches. A conceptual design for a battery-operated, prototype QPAS sensor, which includes 10 pairs of QCLs and tuning forks, would weigh less than 7kg. In addition, the instrument can operate unattended for long periods of time.

QPAS is currently at Technology Readiness Level Five, meaning that while the technical components exist and initial testing is complete, the system still must be converted to a prototype.

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